1. Field of the Invention
The present invention relates to an electromagnet apparatus and an apparatus using the same for adjusting the exposure of a camera, and more particularly to an electromagnet apparatus using a solenoid to convert electrical energy into mechanical energy and an apparatus using the same for adjusting the exposure of a camera.
2. Related Art Statement
An electromagnet typically has a magnetic core made of ferromagnetic material, around which an electromagnetic coil is wound, to utilize a magnetic operation of an electric current flowing through the coil so that mechanical force is generated.
Electromagnets of the foregoing type are classified into a variety of structures exemplified by a solenoid type electromagnet, a clapper type electromagnet, an electromagnet formed by combining the solenoid type and the clapper type, and rotary core type electromagnet.
Among the foregoing electromagnets, the solenoid type electromagnet includes a moving core, the attractive force of which acts on the inside of the electromagnetic coil. The foregoing structure exhibits restricted change in the leakage flux with respect to the stroke as compared with the other structures. Therefore, the attractive force is not changed considerably with respect to the stroke. It leads to a fact that the solenoid type electromagnet can be used suitably for a structure involving a long stroke. The solenoid type electromagnet usually has one pole head for generating the attractive force.
In comparison to the solenoid type electromagnet, the clapper type electromagnet has a moving core disposed on the outside of the electromagnetic coil to face the fixed core. Although the foregoing structure finally generates large attractive force, the leakage flux is enlarged considerably if the air gap is large. Therefore, the attractive force is changed considerably with respect to the stroke. As a result, the clapper type electromagnet is used in a structure involving a short stroke. The clapper type electromagnet has a moving core in the form of a plain head and usually includes two pole heads for generating the attractive force.
The electromagnet formed by combining the solenoid type and the clapper type has a structure that a portion of the clapper-shape moving core is attracted to the inside of the electromagnetic coil. Since the characteristics of the solenoid type structure are added to the moving core, large attractive force can be obtained even if the structure has a long stroke.
The rotary core type electromagnet has a structure that the magnetic pole of the moving core is formed into an unbalanced shape with respect to the center of the magnetic pole. Thus, the attractive force of the rotary core type electromagnet acts as rotary force on the rotational shaft of the moving core.
However, the solenoid type electromagnet has the plunger that moves only along a straight line. Since each of the electromagnets formed by combining the solenoid type and the clapper type and the rotary core type electromagnet has center of rotation orthogonal to the center of winding of the solenoid, there arises a problem in that the structure requires a large space.
There have been suggested a variety of apparatuses for adjusting the exposure of a camera of a type that is operated when electric power is supplied to the solenoid thereof.
For example, shutters each having an arrangement that a straight-moving type plunger solenoid is used as an actuator thereof have been disclosed in Japanese Patent Laid-Open No. 4-194912 and Japanese Patent Application No. 4-089015.
However, the structure disclosed in Japanese Patent Laid-Open No. 4-194912 and having a large-size plunger solenoid encounters a problem that the size of the camera will be enlarged and the size of the camera will be increased if the plunger solenoid is disposed in the body of the camera in a case where the structure is adapted to a camera having a lens barrel that projects over the body of the camera. If the plunger solenoid is disposed in the lens barrel, a similar problem arises in that the size of the lens barrel cannot be reduced and accordingly the size of the camera is enlarged excessively.
Since each of the shutters disclosed in Japanese Patent Laid-Open No. 4-194912 and Japanese Patent Application No. 4-089015 has the structure that the plunger is moved straight, the shutter is undesirably opened when the camera receives an impact. Moreover, the structure of the foregoing shutter having an arrangement that the plunger and the sector are disposed at unbalanced positions causes the shutter speed to be changed due to the dead weight of the plunger and/or the sector, that is a speed difference due to attitude of the camera takes place if the camera is held vertically or laterally.